Radio frame structures and HARQ in a physical downlink used for 3rd Generation Partnership Project (3GPP) Release 8 (referred to as Long Term Evolution (LTE)) and beyond will be described below. Next, carrier aggregation (CA), which has been newly introduced in 3GPP Release 10 (referred to as LTE-Advanced), and its scheduling will be described. Furthermore, a TDD-FDD carrier aggregation which has been newly introduced in 3GPP Release 12 (referred to as LTE-B) will be described.
First, LTE radio frame structures will be described. In 3GPP Release 8 and beyond (i.e., LTE), two types of radio frame structures are specified. One is referred to as a frame structure type 1, which is applied to frequency division duplex (FDD). The other one is referred to as a frame structure type 2, which is applied to time division duplex (TDD). As shown in FIG. 1, in the frame structure type 1 and type 2, the length of one radio frame is 10 milliseconds, and one radio frame is composed of 10 subframes. In the case of TDD, the first 5 subframes (#0 to #4) and the latter 5 subframes (#5 to #9) are each referred to as a half frame. The length of a half frame is 5 milliseconds. The length of one subframe is 1 millisecond. One subframe is divided into two slots each having a length of 0.5 milliseconds. In the case of a normal cyclic prefix, one slot includes seven symbols (i.e., single carrier frequency division multiple access (SC-FDMA) symbols in an uplink, and orthogonal frequency division multiplexing (OFDM) symbols in a downlink) in the time domain. Thus, one subframe includes 14 symbols in the time domain.
FIG. 2 shows seven uplink/downlink configurations (TDD UL/DL configurations) supported by TDD LTE. In the case of TDD LTE, uplink subframes (UL subframes) and downlink subframes (DL subframes) coexist in one radio frame. The TDD UL/DL configuration indicates the arrangement of uplink and downlink subframes in one radio frame. In FIG. 2, “D” represents a DL subframe; “U” represents a UL subframe; and “S” represents a special subframe. The TDD LTE repeatedly uses any one of the TDD UL/DL configurations shown in FIG. 2 in the period of the radio frame (i.e., 10 milliseconds).
The UL subframe is a subframe in which uplink (UL) transmission from a radio terminal (user equipment (UE)) to a base station (eNodeB (eNB)) is performed. The DL subframe is a subframe in which downlink (DL) transmission from a base station to a radio terminal is performed. Switching from DL transmission (DL subframe) to UL transmission (UL subframe) is performed at the second subframe (i.e., subframe #1 or #6) in the half frame. FIG. 3 shows a configuration example of the special subframe. The special subframe is composed of a downlink pilot time slot (DwPTS) in which DL transmission is performed, a guard period (GP) in which no transmission is performed, and an uplink pilot time slot (UpPTS) in which uplink transmission is performed.
Subsequently, the data transmission process in a physical downlink shared channel (PDSCH), including HARQ operation, will be described. In the 3GPP Release 8 and beyond, downlink user data is transmitted on the PDSCH. Meanwhile, control information about downlink communication is transmitted on a physical downlink control channel (PDCCH). The control information about downlink communication includes a downlink (DL) grant indicating allocation of PDSCH resources to the radio terminal. In response to decoding the PDCCH indicating the DL grant, the radio terminal receives downlink data on the PDSCH, checks if there is a cyclic redundancy check (CRC) error in the downlink data, and transmits a CRC result (i.e., acknowledgement (ACK) or negative ACK (NACK)) on a physical uplink control channel (PUCCH) or a physical uplink shared channel (PUSCH).
Upon receiving an ACK, the base station transmits next new data to the radio terminal. Upon receiving a NACK, the base station performs retransmission of the data that has previously been transmitted. Note that the 3GPP Release 8 and beyond supports incremental redundancy (IR) HARQ, and accordingly data to be retransmitted includes additional parity bits different from those in the first transmitted data, which includes systematic bits and some parity bits obtained by turbo coding. Specifically, the base station manages a circular buffer (CB) that stores a code block obtained by performing turbo coding, subblock interleaving and bit collection, and the base station selects an offset (i.e., redundancy version (RV)=0) corresponding to the beginning of the CB for the initial transmission to send the systematic bits. The RV specifies a starting point in the CB to start reading out bits from the CB for transmission. In retransmission, the base station selects a different RV from that used in the initial transmission to perform incremental redundancy (IR) and thus sends additional parity bits which are not included in the initial transmission.
As is well known, the HARQ is a scheme in which forward error correction coding, such as turbo coding, is combined with a primitive ARQ scheme. That is, in the HARQ, user data and CRC bits are protected by error correcting code (ECC). The addition of the error correcting code increases the redundancy and thereby increase the transmission success probability in the HARQ, but it decreases the percentage of the user data within the data to be transmitted (i.e., causes decrease in coding rate). For this reason, the 3GPP Release 8 and beyond supports the incremental redundancy (IR) HARQ, and bit selection and puncturing are performed on the coded code block data stored in the circular buffer during the rate matching process for transmission.
The media access control (MAC) layer in the 3GPP Release 8 and beyond employs the stop-and-wait (SAW) HARQ. Specifically, in the DL transmission, the base station transmits one downlink transport block, and then stops further transmission and waits until a feedback (i.e., ACK or NACK) is received from the radio terminal. Upon receiving an ACK from the radio terminal, the base station transmits a new downlink transport block. Upon receiving a NACK from the radio terminal (or a predetermined period has passed without receiving any feedback), the base station re-transmits the transport block. Such a simple SAW operation reduces the transmission efficiency, which makes it difficult to use the transmission resources (DL radio frames) efficiently. Therefore, the multi-process HARQ is used. In the multi-process HARQ, independent HARQ processes are interlaced in time so that all transmission resources can be efficiently used. Each HARQ process is responsible for a separate SAW operation and uses a separate partition in a soft buffer as described later.
FIG. 4 shows one HARQ process in the downlink in the FDD operation. In the example of FIG. 4, in the subframe #0, the base station transmits a DL grant on the PDCCH and transmits DL data on the PDSCH. The radio terminal receives the DL data on the PDSCH in the subframe #0, decodes the transport block from the DL data, and performs CRC test on the transport block. Further, the radio terminal transmits the CRC result (ACK or NACK) for the transport block, which was transmitted in the subframe #0, on the PDCCH or PUSCH of the subframe #4. The delay time (T_UL_ACK) from the DL data transmission to the ACK/NACK transmission is specified to be four subframes (4 milliseconds).
The base station receives the ACK/NACK from the radio terminal in the subframe #4, and performs re-transmission (in the case of NACK) or transmission of a new transport block (in the case of ACK) in the subframe #m. Note that in the downlink of the 3GPP Release 8 and beyond, asynchronous (asynchronous) HARQ is employed and the retransmission or the subsequent transmission can occur at any time after the initial transmission, and thus the delay time (T_eNB_processing) from the ACK/NACK transmission to the subsequent transmission or retransmission depends on the processing time of the base station. However, it is assumed that the typical length of T_eNB_processing is four subframes (4 milliseconds). Accordingly, in the case of FDD, the typical downlink HARQ round trip time (RTT) is eight subframes (8 milliseconds). Therefore, the 3GPP Release 8 and beyond specifies that the maximum number (MDL_HARQ) of downlink HARQ processes in the FDD operation is 8. As shown in FIG. 4, the HARQ RTT indicates the interval (i.e., T_UL_ACK+T_eNB_processing) from the initial transmission of the DL transport block to the subsequent transmission or retransmission in one HARQ process (SAW operation). FIG. 5 shows a case where eight HARQ processes are used in parallel in the case of FDD. The eight HARQ processes are interlaced in time and operated in accordance with the HARQ RTT of eight subframes.
As described above, in the FDD operation, HARQ-ACK/NACK transmitted in a certain UL subframe #n indicates the CRC result of transmitted data in a DL subframe #(n-4) which is located four subframes in advance. In other words, in the case of FDD operation, the UL subframe for HARQ-ACK/NACK is mapped on a one-to-one basis with the DL subframe which is located four subframes in advance. However, in the case of TDD operation, as is clear from the TDD UL/DL configurations shown in FIG. 2, an UL subframe is not always located four frames after a DL subframe (or a special subframe in which DL transmission can be performed). Further, the arrangement of DL subframes and UL subframes varies depending on the TDD UL/DL configuration. Accordingly, only one type of relationship between the UL subframes and the DL subframes in the FDD operation cannot be applied to the TDD operation. Therefore, each TDD UL/DL configuration has its unique mapping between the UL subframes and the DL subframes for HARQ-ACK/NACK. Further, the HARQ RTT in the TDD operation is generally longer than that in the FDD operation.
Table 1 shows mapping between the UL subframes and the DL subframes for HARQ-ACK/NACK specified for each of the seven TDD UL/DL configurations (see Section 10.1.3.1 of 3GPP TS 36.213 V12.2.0). For example, in the case of the TDD UL/DL configuration 0, HARQ-ACK/NACK transmitted in the UL subframe #2 indicates the CRC result of DL data transmitted in the DL subframe which is located six subframes in advance (i.e., the DL subframe #6 in the previous radio frame). Similarly, in the case of the TDD UL/DL configuration 0, HARQ-ACK/NACK transmitted in the UL subframe #4 indicates the CRC result of DL data transmitted in the DL subframe which is located four subframes in advance (i.e., the DL subframe #0 in the same radio frame). FIG. 6 shows the mapping between the UL subframes and the DL subframes for HARQ-ACK/NACK in the TDD UL/DL configuration 0.
TABLE 1Downlink association for TDDTDD UL/DLSubframe nconfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 6, 4————8, 7, 6, 4——3——11, 7, 66, 55, 4—————4——12, 11, 8, 77, 6, 5, 4——————5——13, 12, 11, 9,———————8, 7, 6, 5, 46———75——77—
As is seen from the mappings used in the TDD UL/DL configurations shown in Table 1, the HARQ RTT in the TDD operation is generally longer than that in the FDD operation. This is because the delay time (T_UL_ACK) from the DL data transmission to the ACK/NACK transmission in the TDD operation is equal to or longer than the delay time (i.e., four subframes) in the FDD operation. As described above, the HARQ RTT (i.e., T_UL_ACK+T_eNB_processing) depends on T_UL_ACK. For example, in the case of the TDD UL/DL configuration 0, the maximum value of the delay time T_UL_ACK is 6. Therefore, assuming that the typical length of T_eNB_processing is four subframes (4 milliseconds), the longest HARQ RTT in the TDD UL/DL configuration 0 is assumed to be 10 subframes (10 milliseconds). Further, in the case of the TDD UL/DL configuration 5, the maximum value of the delay time T_UL_ACK is 13. Therefore, the longest HARQ RTT in the TDD UL/DL configuration 5 is assumed to be 17 subframes (17 milliseconds).
The maximum number (MDL_HARQ) of HARQ processes in the TDD operation should be determined based on the longest HARQ RTT in each TDD UL/DL configuration and the total number of DL subframes and special subframes that exist within the longest HARQ RTT. Therefore, the maximum number (MDL_HARQ) of HARQ processes varies depending on the TDD UL/DL configuration. Table 2 shows the maximum number (MDL_HARQ) of downlink HARQ processes in each TDD UL/DL configuration specified in Section 7 of 3GPP TS 36.213 V12.2.0.
TABLE 2Maximum number of HARQ process for TDDTDD UL/DL configurationMaximum number of HARQ process04172103941251566
Furthermore, the 3GPP Release 10 and beyond specifies the carrier aggregation (CA). In the carrier aggregation, the radio terminal is configured with multiple carriers (referred to as Component Carriers (CCs)) on different frequencies by the base station, and can use these multiple component carriers for uplink communication, downlink communication, or both. Release 10 specifies the carrier aggregation of five CCs at maximum.
The multiple CCs include one primary CC and one or more secondary CCs. The primary CC is also referred to as a primary frequency. The secondary CC is also referred to as a secondary frequency. The primary CC is a CC used for the primary cell (PCell). The primary cell (PCell), which is operated on the primary CC, is a cell in which the radio terminal performs an initial connection establishment, in which the radio terminal performs connection re-establishment, or which is indicated as being a primary cell in a handover procedure. The secondary cell (SCell), which is operated on the secondary CC, is different from the PCell. In general, the secondary cell (SCell) is additionally configured after the radio terminal has established a radio resource control (RRC) connection in the primary cell, and is used to provide the radio terminal with additional radio resources. The radio terminal can simultaneously use multiple serving cells including one primary cell and at least one secondary cell.
Further, in the CA, self-scheduling or cross-carrier scheduling can be used. The self-scheduling is a scheduling method in which a scheduling grant (DL grant or UL grant) is transmitted on the same component carrier as that used by the radio terminal for the DL data reception or the UL data transmission. Cross-carrier scheduling is a scheduling method in which a scheduling grant is transmitted on a different component carrier from that used by the radio terminal for the DL data reception or the UL data transmission. Specifically, in the case of self-scheduling, in order for the scheduling of a certain serving cell to be effective, the radio terminal is configured to monitor a PDCCH transmitted on this serving cell. In the case of cross-carrier scheduling, in order for the scheduling of a certain serving cell (e.g., SCell) to be effective, the radio terminal is configured to monitor a PDCCH transmitted on another serving cell (e.g., PCell).
Note that in 3GPP Release 10, either only FDD component carriers or only TDD component carriers can be aggregated. On the other hand, the 3GPP Release 12 and beyond specifies the CA of an FDD component carrier(s) (FDD CC(s)) with a TDD component carrier(s) (TDD CC(s)). The FDD CC (or FDD cell) is a cell using the frame structure type 1 for FDD. The TDD CC (or TDD cell) is a cell using the frame structure type 2 for TDD. In this description, this type of carrier aggregation is referred to as “FDD-TDD aggregation”, or simply as “FDD-TDD”.
In the FDD-TDD carrier aggregation, the primary cell may be an FDD CC (FDD cell) or a TDD CC (TDD cell). In the FDD-TDD, when the primary cell is a TDD cell and the serving cell (i.e., the secondary cell) is an FDD cell, the maximum number of downlink HARQ processes for this FDD serving cell (secondary cell) is expected to be larger than the value for a FDD cell when the CA is not configured. This is because, when a UL ACK/NACK responsive to DL transmission on the FDD serving cell (secondary cell) is transmitted in accordance with the UL/DL configuration of the TDD primary cell, the HARQ RTT in the FDD serving cell is become larger than when the CA is not configured.
Specifically, 3GPP TS 36.213 V12.2.0 specifies that if the FDD-TDD CA is configured, if the primary cell is a TDD CC (TDD cell) and the serving cell is an FDD CC (FDD cell), and if the self-scheduling is configured for DL transmission in the secondary cell, mapping between the UL subframes and the DL subframes for HARQ-ACK/NACK of the secondary cell shall follow the following Table 3 (see Section 10 of 3GPP TS 36.213 V12.2.0). Note that the “DL-reference UL/DL configuration” in Table 3 indicates the UL/DL configuration of the primary cell.
TABLE 3Downlink association for FDD-TDD and serving cellframe structure type 1DL-referenceUL/DLSubframe nconfiguration01234567890——6, 55, 44——6, 55, 441——7, 66, 5, 4———7, 66, 5, 4—2——8, 7, 6, 5, 4————8, 7, 6, 5, 4——3——11, 10, 9,6, 55, 4—————8, 7, 64——12, 11, 10,7, 6, 5, 4——————9, 8, 75——13, 12, 11,———————10, 9, 8, 7,6, 5, 46——8, 77, 66, 5——77, 6, 5—
For example, when the UL/DL configuration (DL-reference UL/DL configuration) of the TDD primary cell is the configuration 0, in the UL subframe #2, the HARQ feedback (ACK/NACK) for two DL transport blocks, which were transmitted six subframes ago and five subframes ago, is transmitted in accordance with the definition in Table 3. Assuming that the typical length of T_eNB_processing is four subframes (4 milliseconds) as described above, the typical longest HARQ RTT in the FDD serving cell when the TDD primary cell has the UL/DL configuration 0 is 10 subframes (10 milliseconds). Similarly, when the TDD primary cell has the UL/DL configurations 1 to 6, the typical longest HARQ RTTs in the FDD serving cell is 11, 12, 15, 16, 17 and 12, respectively.
Therefore, 3GPP TS 36.213 V12.2.0 specifies that if the FDD-TDD CA is configured and if the primary cell is a TDD cell and the serving cell (i.e., the secondary cell) is an FDD cell, the maximum number of HARQ processes for the serving cell shall be determined in accordance with the following Table 4 (see Section 7 of 3GPP TS 36.213 V12.2.0). Note that the “DL-reference UL/DL configuration” in Table 4 indicates the UL/DL configuration of the primary cell.
TABLE 4Maximum number of HARQ process for FDD-TDD, primary cellframe structure type 2, and serving cell frame structure type 1DL-referenceUL/DL configurationMaximum number of HARQ process010111212315416516612
Section 7 of 3GPP TS 36.213 V12.2.0 specifies the maximum number (MDL_HARQ) of HARQ processes in the other types of CA as follows. In the case of the FDD CA, the maximum number (MDL_HARQ) of HARQ processes per serving cell is 8. In the case of the TDD CA, the maximum number (MDL_HARQ) of HARQ processes per serving cell is determined in accordance with the above-mentioned Table 2 for TDD. If the FDD-TDD CA is configured and if the primary cell is an FDD CC (FDD cell), the maximum number (MDL_HARQ) of HARQ processes per serving cell is 8. If the FDD-TDD CA is configured and if the primary cell is a TDD CC (TDD cell) and the serving cell is a TDD CC (TDD cell), the maximum number (MDL_HARQ) of HARQ processes for the serving cell is determined in accordance with the above-mentioned Table 2 for TDD.
Returning to the description of the downlink HARQ operation, as described above, incremental redundancy (IR) HARQ is used in the downlink. Accordingly, the radio terminal needs to store soft bits (e.g., a log likelihood ratio (LLR)) related to received data, in which a CRC error has been detected, in a memory so as to combine the soft bits with re-transmitted data. This memory is referred to as a “soft buffer” or a “soft bit buffer”. Further, the radio terminal simultaneously manages multiple HARQ processes. That is, the soft buffer included in the radio terminal needs to store the soft bits of the maximum number (MDL_HARQ) of HARQ processes. Therefore, the radio terminal needs to divide the soft buffer based on at least the maximum number (MDL_HARQ) of HARQ processes and secure partitions in the soft buffer for respective HARQ processes.
The method for dividing the soft buffer included in the radio terminal will be described below. The method for dividing the soft buffer is specified in Section 7.1.8 of 3GPP TS 36.213 V12.2.0 and Section 5.1.4.1.2 of 3GPP TS 36.212 V12.1.0. A buffer size nsb per code block is determined in accordance with the following Formulas (1) to (3).
                              n          SB                =                  min          (                                    N              cb                        ,                          ⌊                                                N                  soft                  ′                                                  C                  ·                                      N                    cells                    DL                                    ·                                      K                    MIMO                                    ·                                      min                    ⁡                                          (                                                                        M                          DL_HARQ                                                ,                                                  M                          limit                                                                    )                                                                                  ⌋                                )                                    (        1        )                                          N          cb                =                  min          ⁡                      (                                          ⌊                                                      N                    IR                                    C                                ⌋                            ,                              K                w                                      )                                              (        2        )                                          N          IR                =                  ⌊                                    N              soft                                                      K                C                            ·                              K                MIMO                            ·                              min                ⁡                                  (                                                            M                      DL_HARQ                                        ,                                          M                      limit                                                        )                                                              ⌋                                    (        3        )            
In Formulas (1) to (3), nsb and Ncb each represent the partition size per code block in the soft buffer. NIR represents the partition size per transport block in the soft buffer. N′soft and Nsoft each represent the total size of the soft buffer included in the radio terminal. C represents the number of code blocks into which the transport block is divided. NcellsDL represents the total number of CCs configured in the ratio terminal for CA. KMIMO represents the number of multiple-input multiple-output (MIMO) layers. Kw represents the length of a circular buffer provided in the base station and corresponds to the code block length after performing turbo coding, subblock interleaving, and bit collection. KC is determined by the following: if Nsoft=35982720, Kc=5, else if Nsoft=3654144 and the radio terminal is capable of supporting no more than a maximum of two spatial layers, Kc=2, else Kc=1. Mlimit is a constant equal to 8. MDL_HARQ represents the maximum number of HARQ processes in the serving cell.
N′soft or Nsoft, which is the total size of the soft buffer, is referred to as the total number of soft channel bits, and depends on the capability of the radio terminal (i.e., UE Category). Table 5 shows the total number of soft channel bits (i.e., the total size of the soft buffer) that should be included in the radio terminal for each UE Category, which is specified in Section 4.1 of 3GPP TS 3GPP TS 36.306 V12.1.0.
TABLE 5Total number of soft channel bits set by the UE CategoryUE CategoryTotal number of soft channel bitsCategory 1250368Category 21237248Category 31237248Category 41827072Category 53667200Category 63654144Category 73654144Category 835982720